1. Field of the Invention
The present invention relates generally to optical fiber networks. More particularly, a system and method are disclosed or coupling light from a laser into an optical fiber. "Pigtailing" is the term commonly used to describe the process of aligning and attaching an optical fiber in front of an active optoelectronic device such as a laser diode for a photodiode.
2. Relationship to the Art
Fiberoptic networks are commonly used to transmit both voice and data. A key economic consideration in the design of a fiberoptic network is the maximum length of fiber that may be included between two points before it is necessary to include a signal repeater in the communication path that retransmits the signal. The reason that the signal must be repeated or retransmitted is that as the signal is transmitted along an optical fiber, the signal is attenuated due to the light that carries the signal either leaking out of the fiber or being absorbed by the material from which the fiber is made. When the strength of the signal falls below a certain level as a result of this attenuation, then the signal to noise ratio of the system may become too low for effective data transfer to be maintained.
One way to increase the signal to noise ratio of a transmitted signal is to increase the power of the light transmitted into the optical fiber. For a given amount of attenuation per length of fiber, increasing the power input to the fiber increases the distance that the fiber may be run before the signal level becomes unacceptably small. Of course, one way of increasing the power input to the fiber is to increase the power of the laser used to generate the light that is coupled to the fiber. Another way of increasing the strength of the signal carried by the fiber is to more efficiently couple the modulated light from the laser into the fiber. In general, coupling the light from the laser into a flat-end (as cleaved) fiber is very inefficient, with only about 10 to 15 percent of the laser light output being coupled into the fiber.
The signal to noise ratio of a signal received from the fiber may likewise be increased by increasing the efficiency of the coupling of light from the fiber to a detector used to receive the signal carried by the fiber.
Various techniques have been developed for increasing the amount of light coupled into an optical fiber from a laser diode and of increasing the efficiency of the coupling of light out of an optical fiber to an optoelectronic device such as a detector. For example, lenses are used to gather light from a laser source and focus the light on an end of an optical fiber so that more of the light from the laser is coupled into the fiber. Discrete lenses have been used for this purpose. Lenses have also been formed on the tips of optical fibers.
FIG. 1 is a block diagram illustrating a simplified fiberoptic system 100 for transmitting and receiving. A transmission signal processing block 102 provides a modulation signal containing data to a laser 104. The output of laser 104 is coupled by an optical system 106 into an optical fiber 108. As noted above, optical fiber 108 tends to attenuate the light coupled into it by optical system 106. At the other end of optical fiber 108, the light is coupled by a receiving optical system 110 into a detector 112 that generates a signal for a receiving signal processing system 114.
In general, the light coupled into or out of the optical fiber is highly sensitive to the alignment of the optical fiber with the laser or the detector and any optical system that is used between the optical fiber and such devices. A slight misalignment of the optical fiber may cause a large decrease in the amount of light coupled into the fiber from the laser or out of the fiber to the detector. In general, this problem is more serious at the laser end because the size of the emitting region of a typical laser diode used in a system is approximately 2 .mu.m by 4 .mu.m. In general, the detector is somewhat larger, but the importance of exact alignment is still important. It should also be noted that the use of focusing optics to focus light from the laser into the optical fiber may increase the light coupled into the cable but also increases the sensitivity of the amount of coupling to the alignment of the cable with the source and any discrete optical devices used.
Various techniques have been developed for aligning optical fiber with optoelectronic devices. FIG. 2 is a block diagram illustrating an optoelectronic system 200 that includes a optical fiber end 202 and an optoelectronic device 204. As mentioned above, optoelectronic device 204 may be a laser diode or a detector. Optical fiber 202 is supported by a pedestal 212 and optoelectronic device 204 is supported by a pedestal 214. The pedestals rest on a substrate 210. As mentioned above, aligning and fixing optical fiber 202 is critical to maintaining optimal coupling between optical fiber end 202 and optoelectronic device 204.
Alignment of optical fiber 202 with optoelectronic device 204 may be accomplished by using a micro positioner to change the position of optical fiber 202 while measuring the amount of light coupled from optical fiber 202 to optoelectronic device 204. If, for example, optoelectronic device 204 is a laser diode, the light coming out of the other end of optical fiber 202 may be measured and optical fiber 202 may be positioned so that the amount of light output is maximized. Once optical fiber 202 is properly positioned, it is desirable to fix the cable to pedestal 212 in a manner that maintains the alignment.
Numerous methods of fixing optical fibers to supports have been developed. These methods include using epoxy to glue the fiber to a mount, laser welding the fiber to a mount, and soldering the fiber to a mount. While a certain amount of success has been enjoyed using each of those methods, improved performance, especially under varying temperatures is desired, and each of the methods mentioned currently have drawbacks. For example, when epoxy is used to glue the optical fiber to a mount, outgassing and softening of the epoxy has been a problem. As a result, epoxy is seldom used to attach optoelectronic fibers in telecommunication applications. Epoxy has had some use in fiberoptic local area network applications, however.
Another method, laser welding, is commonly used to fix optical fibers to mounts. A joint is formed using a metal to metal weld using a high power laser. For example, a fiberoptic support clip that is laser welded to a sleeve that holds a optical fiber is disclosed in U.S. Pat. No. 5,619,609, issued to Pan et al. which is herein incorporated by reference for all purposes. Pan et al. teaches a special clip that includes a channel having sides that are laser welded to the sleeve which holds the optical fiber. Although such laser welding techniques have proven useful, laser welding equipment is both expensive and difficult to configure for use in fiber pigtailing. In addition, a relatively large amount of energy is delivered during laser welding and the thermal shock that results tends to alter the alignment of the optical fiber during the welding process. Various methods have been designed for causing the incident laser energy to be symmetrically delivered to try to cancel out these effects.
An alternative method is soldering. One preferred soldering method is laser soldering. In laser soldering, an infrared laser is used as a heating source to melt solder that is used to fix a fiber or other device in place. The fiber or device must be metallized for solder to be applied.
The amount of energy delivered during a laser soldering process is much less than the energy required for laser welding. For example, some laser soldering processes use about five watts of energy, which is about a factor of ten less than the amount of energy required for laser welding. A solder preform used to fix a optical fiber is described in U.S. Pat. No. 5,692,086, issued to Beranek et al., which is herein incorporated by reference for all purposes. In general, solder may be used to fix a optical fiber and either laser energy or a more conventional hot plate may be used to melt the solder and fix the optical fiber. However, the use of high temperature solder is generally preferred to provide a solder joint of the desired strength and this may cause difficulty in the soldering hierarchy of the optoelectronic system if a hotplate is used. Therefore, laser soldering is preferred, since high temperature solder may be used.
Although good results have been realized using laser soldering, misalignment tends to occur with variation in temperature since the solder generally has different thermal characteristics than the other materials that are included in the optoelectronic packaging. Misalignment is particularly acute when high temperature solder is used. There is a need, therefore, for designing methods of decreasing misalignment in systems that use solder to fix an optical fiber in alignment with an optoelectronic device. There is especially a need to improve the performance of such systems under varying thermal conditions.